In devices that generate heat such as in an LED, one such device has a mount structure in which two members, which are a substrate and a heat dissipation section, are joined for transporting heat from the substrate on which elements are mounted to the heat dissipation section for the purpose of dissipating generated heat.
In recent years, there is an increasing amount of heat generated by increasingly high output devices such as the LED. Accordingly, it is required to dissipate heat more efficiently. In order to dissipate heat efficiently, there is an increase in area of a joining portion between members of a mount structure. As a path for dissipating heat can be increased by increasing the area, larger output can be realized.
As a joining material for the joining portion of the mount structure, it is possible to use a solder material at a low temperature.
As the temperature is changed due to heat generation when the mount structure is operated, thermal stress and thermal strain are generated due to a difference in linear expansion coefficients between members at the time of temperature change in a case where materials of two members differ.
However, the thermal strain and the thermal stress are increased by the increase in area of the joining portion. Therefore, in a case where the temperature change occurs in the joining portion repeatedly, there is a problem of difficulty for a solder joining portion to withstand the changes and cracks occur. The occurrence of cracks causes a reduction in the heat dissipation property and a reduction in the joining strength. A mount structure which can withstand the development of cracks due to repeated temperature changes is required even when the joining portion with a large area exceeds 10 mm square.
Accordingly, as a related-art mount structure in which development of cracks hardly occurs, there is a mount structure in which a ceramic substrate has a metallized film, a metallic support that is connected to the metallized film through a stress relaxation material, a portion between the metallized film and the stress relaxation material is connected by a first joining material, a portion between the stress relaxation material and the metallic support is connected by a second joining material and the first joining material is thicker than the second joining material (for example, refer to Japanese Patent No. 2566341 (Patent Literature 1)).
However, in the mount structure described in Patent Literature 1, it is necessary to form the first joining material to be thicker than the second joining material, and the ceramic substrate tends to be inclined due to a difference of wettability between the first joining material and the second joining material at the time of mounting. This is a problem because the inclination of the ceramic substrate affects a radiation direction of light in devices such as the LED. Moreover, stress concentration occurs in the first and second joining materials when mechanical properties between the stress relaxation material and the first/second joining materials differ. Therefore, cracks easily occur, and crack formation is more prominent when the joining portion is increased in area. In particular, it is difficult to suppress development of cracks in first and second joining portions by using a Sn—Pb based solder with low bearing strength in the mount structure as described in Patent Literature 1.